The present invention is directed to process variable transmitters. More specifically, the present invention is directed to a process variable transmitter having a selectively adjustable operating range.
2. Description Of The Prior Art
Process variable transmitters, such as pressure transmitters, are factory calibrated during their manufacture to produce a "zero" output of a preselected current value, e.g., 4 ma., at a zero fluid pressure input and a full scale output of a second preselected current value, e.g., 20 ma., at a fluid pressure input other than zero called the upper range limit or URL. It is often desired to change the range of the transmitter to satisfy the measurement and requirements of specific applications by the customer in which the transmitter is to be used. Such an adjustment enables the transmitter to measure an applied input, e.g., a fluid pressure input, that is within its lower range value (LRV) and upper range value (URV) and to produce an output, e.g., between 4 ma. and 20 ma., which is a transmitter output signal proportional to its process variable input between the LRV and URV. The upper and lower range values are adjusted to change the range of the transmitter such that the LRV occurs at some pressure other than a zero fluid input pressure and the URV output occurs at some pressure other than the upper range limit (URL). In conventional devices, the range is changed by manually adjusting circuit potentiometers which set an amplifier gain and zero offset. Specifically, a pressure equal to the desired LRV is applied to the transmitter, and the zero adjust potentiometer is manipulated until the output of the transmitter is measured to be at the lower limit, e.g., 4 ma. A pressure equal to the desired URV is then applied to the transmitter, and the span (gain) adjust potentiometer is manipulated until the output is measured to be at the upper limit, e.g., 20 ma. In this adjustment procedure, the zero and span adjustments interact, i.e., changing one affects the other. For this reason, the aforesaid procedure must be repeated until the amount of adjustment required at either point is insignificant, i.e., does not significantly affect the calibration of the other point. Accordingly, it would be desirable to provide a process variable transmitter which has a selectively variable operating range that is set by a single adjustment operation.